Generally, a solid electrolytic capacitor is obtained by (i) subjecting the surface of an anode body of a valve-action metal such as aluminum, tantalum, niobium, titanium or alloys of these metals to make the surface rough with micropores on the order of microns formed thereon to thereby increase the surface area, (ii) forming a dielectric oxide film thereon through chemical formation, (iii) impregnating a solid electrolyte via a separator between the film and the anode part or forming a solid electrolyte layer therebetween, (iv) forming a cathode electroconductive layer from carbon paste and a metal-containing electroconductive paste thereon, (v) welding the body to a lead frame which serves as an external electrode and then (vi) forming an outer casing of epoxy resin or the like.
Particularly, since solid electrolytic capacitors using as solid electrolyte electroconductive polymers can have reduced equivalent series resistance and leakage current as compared with solid electrolytic capacitors using manganese dioxide as solid electrolyte, they are useful as capacitors meeting demands for higher performance and downsizing of electronic devices and many production methods have been proposed.
When a high-performance solid electrolytic capacitor is produced by using an electroconductive polymer, particularly in case of using a valve-action metal foil, it is indispensable to ensure electrical insulation between anode part serving as anode part and cathode part consisting of electroconductive layer containing electroconductive polymer. However, in a step of impregnating or forming solid electrolyte, it sometimes occurs that solid electrolyte intrudes into the anode region, so-called “creeping up”. In such a case, insulation failure is caused between the anode part and the cathode part.
Examples of shielding measure for insulating the anode part of solid electrolytic capacitor from the cathode part include a method where after a polyamic acid film is formed by allowing a solution containing polyamic acid salt to electrodeposit on at least one part of valve-action metal having no solid electrolyte formed thereon, a polyimide film is formed by dehydration and curing with heat on the part (Patent Document 1: Japanese Patent Application Laid-Open No. H05-47611), and a method for producing solid electrolyte comprising a coating step of a masking material solution which infiltrates into a dielectric film of a solid electrolytic capacitor and forms a masking layer on the infiltrated part (Patent Document 2: International Publication No. WO00/67267 pamphlet (U.S. Pat. No. 6,890,363)).
Generally, various additives for modifying the shielding layer are added to the masking material solution in order to improve bonding to another substrate, the surface conditions and leveling property. For example, a polyimide precursor composition having a high concentration and a low viscosity which can give a heat resistant polyimide film is disclosed (Patent Document 3: JP-A-H10-182820) (U.S. Pat. No. 5,891,986), to which a surface tension-controlling agent and a thixotropic agent can be incorporated as a preferred embodiment.
As a surface tension-controlling agent, a silicone surface tension-controlling agent such as silicone oil, a non-silicone surface tension-controlling agent such as a higher fatty acid ester of glycerol, a borate of a higher fatty alcohol and a fluorine-containing surface active agent can be suitably used, and it is known that the surface tension-controlling agent can be incorporated in an amount of 0.01 to 1 mass % (to the mass of the masking material).    Patent Document 1: Japanese Patent Application Laid-Open No. H05-47611    Patent Document 2: International Publication No. WO00/67267 pamphlet    Patent Document 3: Japanese Patent Application Laid-Open No. H10-182820